1. FIELD OF THE INVENTION
This invention relates generally to a system for performing computed x-ray tomography and more particularly to an arrangement for precisely sensing and indicating that a patient extends beyond an image reconstruction circle of the system.
2. DESCRIPTION OF THE PRIOR ART
In one method of computed tomography a patient is supported for being translated along a longitudinal axis which is usually horizontally disposed. The axis coincides with the center of rotation of a rotatable gantry which has an x-ray source on one side of the center of rotation and a multiple array of x-ray detectors on the other side. A fan shaped x-ray beam that is thin in the longitudinal direction is projected through the patient as the gantry rotates so that the detectors may develop signals indicative of x-ray transmission characteristics along a plurality of paths through the patient undergoing examination. Analog signals representative of x-ray attenuation by all of the volume elements in a layer of the patient at various rotational angles are then converted to digital signals which are used by a computer to produce signals which may thereafter be used to produce a reconstructed image of the layer. The image is reconstructed by analyzing all of the signals within a central cross-sectional area of the system known as the "reconstruction circle." The analog signals are analyzed and converted based upon all of the x-ray attenuation taking place within the reconstruction circle. If the cross section of the patient is either too large or improperly positioned so that a portion of the patient extends outside of the reconstruction circle, the x-ray attenuation of that portion will be erroneously interpreted. The resulting reconstructed image might not be clear or accurate and could require reexamination, or could even provide misinformation. It is therefore extremely important to know, before the scanning process, whether the patient is properly positioned within the reconstruction circle to alleviate the necessity of re-examination.
One method of sensing an oversized or improperly positioned patient involves the application of mechanical templates and sizing devices physically over and around the patient while the patient is in the staged position. Such devices are awkward to handle, time consuming and are typically removed prior to transaltion of the patient into the system. The patient-oversize checks by this method are not always performed closely adjacent to the scan plane. There is time for the patient to shift out of position, between the time of checking and the time of scanning.
A problem associated with the prior art is that the oversized patient check is not done automatically and can be overlooked. Another problem is that there is no opportunity to monitor the patient once in the gantry and between successive scans to insure that the patient remains within the reconstruction circle.
There are situations where the scans will be desired by the physician or radiology technician even though the patient may extend beyond the reconstruction circle. It is important however that these situations always be acknowledged and recorded for proper caution in interpretation of the scans.
Accordingly, one object of this invention is to provide an arrangement to automatically sense and indicate that a patient extends beyond an image reconstruction circle of a computed tomography system.
Another object is to monitor the size and position of the patient over the area to be examined immediately prior to initiating the scanning operation.
Another object is to monitor the size and position of a patient by an arrangement which does not interfere with the patient or the scanning operation and is immediately adjacent to the scan plane of the system.
Another object is to indicate an oversized patient condition on a control panel of the system.
Still another object of this invention is to provide an arrangement to indicate the presence of an oversized patient on an operator's console which requires an operator response, such as, to either abort the scan or override and continue with the examination.